The pathogenesis of ischemic brain injury remains poorly understood. Brain lactate content is directly proportional to the severity of ischemic brain damage after complete ischemia and implies that acidosis can irreversibly injure brain. However, the molecular mechanisms of the H+ induced injury remain incompletely understood. Our studies suggest that under ischemic conditions which can evolve to infarction, the generation of excess H+ remains confined to a brain space consistent with glia. Such compartmentalization for H+ may be a result of altered properties of glial cell membranes. The contribution of brain cell membranes to H+ homeostasis during ischemia has not been emphasized. A hypothesis is developed based on in vivo recordings that brain infarction from ischemia occurs because of severe acidosis greater than 5.2 pH in glia. Furthermore, this acidosis is likely to result from continued glial lactic acid production coupled to loss of intracellular bicarbonate stores and failure of plasma membrane antiport systems for H+ transport but retained plasma membrane integrity. We plan to examine H+ homeostasis in mammalian neurons, glia, and their interstitial microenvironment. Pairs of H+ selective microelectrodes will be used to simultaneously monitor interstitial and selected intracellular (H+) as well as cell membrane electrical characteristics under ischemic conditions. H+ in ischemia may, in addition to a direct toxic effect, injure brain indirectly through increased modality, loss of cell volume regulation, and resultant postischemic lethal brain edema. Therefore, we will also correlate changes in brain H+ homeostasis to these latter variables of tissue modality, lactate content, and per cent swelling. Cell injury will be assessed by changes in cell electrical characteristics, trans-membrane ion gradients, and visualized by light microscopic techniques. Cells will be identified by their evoked membrane electrical characteristics and through selected horse radish peroxidase staining. The general objective of this study is to characterize the patterns and mechanisms of H+ regulation in mammalian brain cells and their interstitial microenvironment under normal and ischemic conditions so as to test the hypothesis that inhibition of plasma membrane H+ regulatory mechanisms can lead to irreversible dysfunction of glial cells and subsequent brain infarction.